The present invention relates to an auxiliary power unit, and more particularly to a multi-spool auxiliary power unit having a governed high-speed spool, which drives a generator at a constant speed to provide consistent electrical power under variable load conditions.
An auxiliary power unit (APU) system provides auxiliary and/or emergency power to one or more aircraft loads. In conventional APU systems, a dedicated starter motor is operated during a starting sequence to bring a gas turbine engine up to self-sustaining speed, following which the engine is accelerated to operating speed. Once this condition is reached, a generator is coupled to and driven by the gas turbine engine during operation whereupon the generator develops electrical power.
The APU must provide constant electric power over the full range of flight speed, altitudes, ambient temperatures and other conditions. Constant mechanical/electric power requirement typically drives the APU thermodynamic power to large values at high altitudes, which requires relatively large aerodynamic components. The components size, weight and fuel burn are minimized by running more than one spool to higher than usual APU cycle pressure ratio.
In conventional multi-spool APUs, the generator is located on the low-pressure (LP) spool, along with the load compressor. This arrangement is vulnerable to spool speed and generator frequency swings under transient electrical loads, i.e. instantaneous step-like off-loads and on-loads. The impact of such varying electrical loads results in varying shaft speed and varying net electric power output frequency. Variability of power output frequency more than +/xe2x88x920.5% is considered a compromise of power quality.
Various attempts have been provided to govern the LP spool to a constant speed. However, it is difficult to maintain constant LP shaft speed and frequency under electrical step load changes of considerable magnitude as the high-pressure (HP) spool inertia slows the speed change needed to meet the load demand. The LP spool consequently tends to over-speed in off-load, and under-speed on load increase.
Accordingly, it is desirable to provide an APU which provides consistent electrical power under variable load conditions such as instantaneous step-like electrical off-loads and on-loads.
The APU system according to the present invention provides a gas turbine engine having a low-pressure spool, a high-pressure spool and an electrical generator. The electrical generator is driven by the high-pressure spool. Preferably, the electrical generator is a starter generator which operates as a starter to spin up the high power spool to light-off the gas turbine engine.
A control valve is operated by a controller in response to the electrical loads currently powered by the generator to drive the high pressure spool at a constant speed. The high pressure spool is preferably driven at a constant speed to provide a specified generator frequency, such as 400 Hz. With the high pressure spool governed at the constant speed, generator step load changes are met by sharply changing the fuel flow onlyxe2x80x94which is very nimble.
Conversely, the low pressure spool is not governed at all, but is allowed to seek a speed that balances the power developed by the low pressure turbine (LPT) and power absorbed by the low pressure compressor (LPC). The low pressure spool shaft speed increases on increasing power demand, and decreases on reduced power demand. The spool has significant mechanical inertia and time is needed to achieve the high pressure spool power balance.
Generally, at lower altitudes the high pressure turbine inlet temperature (TIT) is relatively low, causing the low pressure spool to run relatively slower. At higher altitudes the ambient air pressure and density fall causing the TIT to be driven upward to maintain the high pressure spool shaft speed at given generator load. Concurrently, extra energy in the hot expansion gas drives the low pressure spool faster, which increases the airflow and APU power, until LP speed, airflow, fuel flow and generator power are balanced. Thus, by pumping more air at increasing shaft speed the low pressure spool flattens out the TIT increase requirement by increasing the airflow. That is, as power generated by the spool is essentially airflow rate flow multiplied by TIT, an increase in airflow rate results in a decreased requirement for an increase in TIT.
In operation, a step increase/decrease in electrical power demand is met with a step increase/decrease in fuel flow, which results in an overshoot/undershoot of the new equilibrium TIT. The TIT is restored to the new equilibrium when the LP spool has achieved its new shaft speed and new equilibrium power balance. The HP spool and generator maintain essentially constant speed and frequency, while the LP spool responds in a following manner to restore equilibrium. The APU""s response to sudden power change is thereby effectively immediate as the power change is decoupled from the large mechanical inertia of the HP and LP spools.
The present invention therefore provides consistent electrical power under variable load conditions such as instantaneous step-like electrical off-loads and on-loads.